In employing a deployable trailing axle with steerable wheels to extend the load capacity of a motor vehicle by reducing the loads that the other axles support, conventional practice is mainly concerned with the load distribution between all the axles supporting the vehicle and accommodating differences in road grade between the trailing axle wheels and the wheels of the vehicle's primary axles that normally support the vehicle. As well as isolating shock forces on the steerable trailing axle wheels from the vehicle when the wheels encounter bumps and dips either conjointly or separately. While on the other hand, less concern is addressed to the extent to which a trailing axle suspension system can contribute to enhancing the roll stability of the vehicle. And this applies to a wide range of motor vehicles and especially payload-carrying motors such as refuse trucks, dump trucks, transit mixers and the like where a trailing axle can significantly extend the payload capacity while meeting road restrictions governing axle loading and the wheel base required of the vehicle in meeting bridge restrictions.
In addressing the matter of enhancing the vehicle roll stability with a trailing axle suspension for a payload-carrying vehicle, a major challenge is presented in providing such where the center of gravity of a load can vary significantly from a low elevation on the vehicle with little or no added load to a high elevation on the vehicle at the maximum allowable load weight. And as a result, centripetal forces resulting from the vehicle negotiating a curve at considerable speed and crosswind forces as well as unbalanced lateral forces acting on the vehicle can induce rolling/tilting of the vehicle frame to a varying degree that can be difficult to control. Especially when an added load is the maximum allowable with its center of gravity thus located at a high elevation relative to the vehicle frame's roll axis where it can have a large influence on vehicle roll resulting from centripetal forces and other forces acting to tilt or roll the vehicle frame about its roll axis. And this can include when the vehicle is encountering high crosswinds or executing a quick directional change even at a relatively low speed.
Moreover, it will be appreciated that a trailing axle in reducing the load on the drive axle(s) also reduces the roll stiffness of the drive axle suspension system despite the fact that there is actually a significantly heavier load with an accompanying higher center of gravity. With the trailing axle thus causing the drive axle suspension system to operate as though the load weighs significantly less than it actually is rather than increasing in roll stiffness to the degree that it otherwise could without being relieved of load to a certain degree by the trailing axle.
However, in attempting to maximize the degree to which a trailing axle suspension system can contribute to vehicle roll stability while allowing the axle to adjust to relatively large road grade differences between the trailing axle wheels and the wheels of the primary axles and blocking shock forces on the trailing axle wheels from the vehicle depends on whether the trailing axle is suspended from either the tailgate or the vehicle body or directly from the vehicle frame supporting the vehicle body. As they each present different challenges in attempting to arrive at a trailing axle suspension system that significantly contributes to vehicle roll stability without compromising other possible attributes of the trailing axle suspension system.
For example, it has been found that in the case of suspending a trailing axle with steerable wheels from a tailgate of a vehicle body that may not need to be tilted to discharge a load, there is the matter of then accommodating the axle in a suitable stowed condition without accompanying undesirable consequences. While significantly increasing the contribution of the trailing axle suspension to vehicle roll stability when the trailing axle is deployed and in also isolating shock forces on the trailing axle wheels from the tailgate. On the other hand, it has been found that in the case of suspending a trailing axle with steerable wheels from a vehicle body that is tilted to discharge a load, there is the matter of accommodating the trailing axle in a suitable stowed condition on the vehicle body and the operation of its tailgate while significantly increasing the contribution of the trailing axle suspension system to vehicle roll stability when the trailing axle is deployed and in also isolating shock forces on the trailing axle wheels from the vehicle body. And in the case of suspending the trailing axle directly from the vehicle frame and in seeking enhanced vehicle frame roll stability with the trailing axle, there is the matter of typically having to add further frame structure for suspending the trailing axle clear of the vehicle body to accommodate the operation of its tailgate and regardless of whether the vehicle frame that supports the vehicle body is tilted or not to discharge a load with the tailgate open. Moreover, suspending the trailing axle from the frame normally requires significantly more lateral (anti-roll) force derived from the trailing axle via the actuators to counter the roll inducing forces on the frame.